The present invention relates to a method for metering a reducing agent, especially urea, respectively, aqueous urea solutions, into NO.sub.x -containing exhaust gasses of an internal combustion engine into the exhaust gas line upstream of a catalytic convertor having at least NO.sub.x reducing activity and having a minimal heat capacity and no substantial reducing agent storage activity.
In the selective catalytic reduction (SCR) of NO.sub.x, a certain amount of reducing agent must be metered into the exhaust gas stream depending on the operational state of the internal combustion engine and of the SCR catalyst in order to reduce NO.sub.x to N.sub.2. When not enough reducing agent is metered into the exhaust gas, the degree of reduction decreases unnecessarily. When too much reducing agent is metered into the exhaust gas, slip of reducing agent will be caused as well as undesirable intermediate and cleavage products. Furthermore, an unnecessarily high use of reducing agent will result. When the reducing agent is urea, this means that ammonia breakthrough or, when an oxidation catalytic converter is arranged downstream of the SCR catalytic convertor, emission of dinitrogen oxide (N.sub.2 O) will result.
From European patent application 0 515 857 A1 as well as European patent application 0 697 062 B1 methods are known which are based on the storage capacity of the catalytic convertor and an overstoichiometric amount of reducing agent being introduced intermittently.
In German patent application 43 10 961 A1 a method for detecting the catalyst activity by use of at least one temperature sensor within the catalytic convertor is disclosed.
All known methods have in common that no exact metering of reducing agent into the exhaust gas under dynamic or highly dynamic operating conditions of the internal combustion engine is possible so that dynamic or highly dynamic changes have been ignored in the past. Especially, the precise determination of the catalyst activity during dynamic load changes of the internal combustion engine has been an unsolved problem in the past.
Moreover, only a few relevant parameters have been taken into consideration in the known reducing agent metering methods. In the method known of German patent application 43 10 961 A1 it is suggested to position a plurality of temperature sensors into the catalytic convertor in order to determine one of the parameters of the catalytic convertor, i.e., the temperature. However, this appears to be not very practical and is also very expensive.
An exact determination of the reducing agent filling degree within the SCR catalyst, as is necessary in order to realize reducing agent metering according to the suggestions of European patent applications 0 697 062 B1 and 0 515 857 A1, has not been possible in the past due to the complex interrelationships of such catalyst systems. For this reason, the addition of reducing agent is intermittent and the catalyst is in this manner employed until it is empty in order to reinstate defined conditions. The disadvantage is that the NO.sub.x conversion is thus lowered.
The aforementioned documents thus are based on a metering strategy depending on a considerable storage capacity of the catalytic convertor with respect to reducing agent and heat.
This approach is, in principle, realizable for full (solid) catalytic convertors as known from power plant technology. They are comprised entirely of active material that allows, especially at low temperatures, storage of reducing agent. Due to this storage capacity the reducing agent breakthroughs at dynamic or highly dynamic load changes can thus be almost entirely avoided because excess reducing agent, especially NH.sub.3, can be stored within the catalyst for a short period of time.
However, the use of such full catalytic systems in spatially limited conditions, for example, in vehicles, where a considerable reduction of dimensions and weight of the available systems is required, is not feasible because there is insufficient development potential with regard to the aforementioned specifications.
For this reason, coated catalyst are used having a cell number that is considerably higher and thus having a considerably reduced wall thickness of the support structure and a larger free flow surface with reduced pressure loss, see FIG. 6.
Such a catalyst system, however, has a minimal storage capacity for NH.sub.3 because active material of a substantially reduced activity is available. Reducing agent peaks thus cannot be compensated by absorption, and undesirable emissions of reducing agent products is thus a great risk.
Accordingly, greater demands with respect to precision of metering and the detection of parameters, such as catalyst temperature, NO.sub.x concentration, exhaust gas volume, catalyst activity etc., is much more important in comparison to full catalytic systems.
It is therefore an object of the present invention to provide a method of the aforementioned kind with which dynamic changes of the internal combustion engine operation can be taken into consideration in order to optimize the amount of reducing agent supplied to the exhaust gas.